Protocol 1, Geneva Convention 1977, Art 35. Basic rules
"1. In any armed conflict, the right of the Parties to the conflict to choose methods or means of warfare is not unlimited.
2. It is prohibited to employ weapons, projectiles and material and methods of warfare of a nature to cause superfluous injury or unnecessary suffering.
3. It is prohibited to employ methods or means of warfare which are intended, or may be expected, to cause widespread, long-term and severe damage to the natural environment."
The Protocol also prohibits attacks against the environment by way of reprisals.
See also the definition of a war crime in Rome Statute of the International Criminal Court
"Environmental peacebuilding integrates natural resource management in conflict prevention, mitigation, resolution, and recovery to build resilience in communities affected by conflict."
The contents of the web site below were produced as part of a broad collaboration led by Environmental Law Institute, UNEP, McGill University, and the University of Tokyo, together with 225 researchers and practitioners around the world. Chapters from published works become available as free downloads 6 months after publication.
Further guidance on environmental law generally
Subject searches to try in SOLO
War -- Environmental aspects
Environmental law, International;
War (International law);
Official Paper items are available on the Ground Floor of the Law Library (see example below). SOLO searches will probably also reveal useful commentary held in the Bodleian Social Science Library (SSL) which is in the neighbouring building to the Law Library